This invention relates to an improved apparatus and process for withdrawing agglomerated solids from a fluidized bed of finely divided solid hydrocarbonaceous material and more particularly, to an apparatus and process wherein the agglomeration is effected by a high temperature reaction between the inorganic constituents of the hydrocarbonaceous material in the fluidized bed environment.
A coal gasification reactor wherein agglomerated coal ash is withdrawn from a fluidized reaction bed of finely divided coal without the removal of the finely divided coal particles is described in Jequier et al, U.S. Pat. No. 2,906,608, the teachings of which are incorporated by reference herein. In a coal to gas conversion process of the type described in Jequier et al, a vessel is provided to contain a fluidized bed. A gas distribution grid is usually positioned in the bottom of the vessel to define the bottom surface of the fluidized bed. The central portion of the grid may be conical or cylindrical in shape and comprises a downwardly extending passage. At the bottom of the passage, a constriction is provided having an opening defining a venturi of fixed throat size to guide the gas passing into the vessel upwards into the fluidized bed. A stream of high velocity gas is directed through the venturi or passage into the reaction vessel, causing ash particles to agglomerate in the bottom portion of the vessel and eventually discharge downwardly through the passage and venturi throat.
In Jequier et al, at column 3, lines 7-10, it is taught that "[i]t is desirable that the gasification agents should have a speed perpendicular to the smallest section of the venturi and at this point they should not have any helicoidal movement." It has been discovered that a direct upward passage of fluid through the venturi does not provide the most efficient rate of solid discharge through the venturi. Improved solids discharge or removal can be obtained through provision of ridges or other devices to permit the descent of agglomerated ash.
In Schora et al, U.S. Pat. No. 4,023,280 and Patel et al, U.S. Pat. No. 4,057,402, the environment in which the claimed invention could operate as well as some of the problems associated with a classifier for a fluidized bed ash agglomerating gasifier are also described. With respect to Schora et al, an apparatus and method for varying the cross-sectional area of the venturi input in a fluidized bed retained within a vessel are disclosed. With respect to Patel et al, a process for gasifying carbonaceous feed materials is disclosed. Both patents utilize a venturi which has a smooth internal surface which provides for a combination of agglomerate release but at undesirably low agglomerate flow rates and evidencing incomplete separation of ash from unreacted hydrocarbonaceous material, such as char.
Keyser, U.S. Pat. No. 3,636,983, Wilis, U.S. Pat. No. 3,693,329 and Galliot, U.S. Pat. No. 2,017,043, describe fluid flow arrangements in hollow pipes in which a spiral angular velocity is imparted to the fluid by the use of ridges disposed within a pipe. None of the Keyser, Willis or Galliot patents contemplates the passage and agglomeration of particulate matter, such as agglomerated ash, falling by force of gravity countercurrent to a high temperature gas flow, nor the problems associated with ash agglomeration as encountered by applicant.
A problem associated with the apparatus as illustrated in Jequier et al is that extremely high temperatures are present in the defined conical withdrawal section. In addition, since the abrasive agglomerated ash particles are in constant physical contact with the walls of the cone and because of the high temperatures present therein, expensive alloys are required to manufacture a long lasting withdrawal cone. More importantly, since the gas stream that surrounds the ash agglomerates is the same as the stream separating or classifying the agglomerates from the fluidized bed, unusual restrictions are imposed on the rate and composition of gas flow. In addition to undesirable sintering taking place in the venturi, the nozzle can plug, particularly when fine coal material, as recovered from product gases, is recycled back to the fluidized bed through the venturi nozzle. Because the plugging occurs in a confined zone of high temperature, a fused adherent mass, if it forms, can lead to an undesired premature reactor shutdown. Problems such as these have created a need for modifications in the classification apparatus.
The use of any interior ridges, spiral or otherwise, in a fluidized bed gasifier venturi to facilitate ash agglomeration and selective solids withdrawal has not been disclosed in any of the prior art. The smooth bore venturi orifices used in the past have relatively low rates of solid discharge from fluidized bed gasifiers and incomplete separation and classification of ash from unreacted solid hydrocarbonaceous material, such as char. It is with this background that the present invention was developed.